This proposal seeks to explain how the lac repressor recognizes the unique base sequence of the lac operator. Alone, this protein binds specifically and very tightly to the lac operator, but a variety of galactosides induce it to no longer bind. These inducers must alter features of the protein which are essential for its DNA binding activity. Two types of change are considered: 1) A conformational change which destroys the operator binding site, and 2) A change in the arrangement of two or more identical subsites which prevents them from simultaneously binding to a single operator. Adler et al. have proposed that tyrosine 17 binds directly to DNA. We will identify tyrosines which bind to DNA by their exposure to nitration with tetranitromethane in the absence but not the presence of DNA. The 3-nitrotyrosines introduced will be reduced and preferentially acylated with fluorescent probes, e.g. dansyl chloride, which will detect changes in the polarity and rotational mobility of the operator site when inducers bind. The kinetics of any change will be investigated by stopped-flow fluorimetric methods. The absence of any change and the presence of subsites would suggest that inducer binding affects the arrangement of these subsites.